Well, do to the random nature of the items in the game, items can have more value to someone who want to do the IDing and hoping for a great roll.
example, a manticore cross bow can roll with 2 sockets but you won't know until you id it. 1 socket and junk mods are not worth much, 2 sockets and good mods is worth a lot. unid is worth more than the former and less than the latter.

There are many avenues to trade UNID items. If you are generally new to D3 this can be a quick source of $$ to start to improve your character. The downside is, as Spiffy states, if the item rolls amazing then you lose out on the big money because you sold it UNID. It is always a crap shoot.